MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 156 AM EST FRI JAN 6 2017 VALID JAN 6/0000 UTC THRU JAN 9/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z FINAL MODEL EVALUATION INCLUDING PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ SHORTWAVE TROUGH OVER THE NORTH-CENTRAL U.S. THROUGH FRIDAY ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: GENERAL MODEL COMPROMISE CONFIDENCE: AVERAGE IN THE WAKE OF THE EXITING UPPER LOW OVER EASTERN CANADA, BROAD TROUGHING REBUILDS ACROSS THE NORTHERN TIER OF THE U.S. WITH AN ELONGATED WEST-EAST ORIENTED SHORTWAVE. THIS VORTICITY WILL THEN LIFT NORTHEAST ACROSS NEW ENGLAND BY FRIDAY NIGHT. THIS FEATURE WILL ALSO SUSTAIN A SURFACE LOW JUST OFF THE MID-ATLANTIC COAST THROUGH MIDDAY FRIDAY BEFORE GOING OUT TO SEA. GOOD OVERALL MODEL AGREEMENT EXISTS AND THEREFORE A GENERAL MODEL COMPROMISE CAN BE INCORPORATED INTO THE FORECAST. PACIFIC NORTHWEST TROUGH TRACKING TO THE EAST COAST SATURDAY ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z GFS/00Z ECMWF CONFIDENCE: SLIGHTLY BELOW AVERAGE THE MID-LEVEL TROUGH CURRENTLY CROSSING THE INTERMOUNTAIN WEST WILL CONTINUE MOVING TOWARDS THE SOUTHEAST AS THE BLOCKING PATTERN OVER THE GULF OF ALASKA BREAKS DOWN FURTHER AND A LARGE UPPER LOW DEVELOPS (DESCRIBED IN LAST SECTION). THIS WILL QUICKLY PROGRESS TO THE EAST COAST BY SATURDAY AND INDUCE SURFACE CYCLOGENESIS OVER FLORIDA AND OFF THE SOUTHEAST COAST. THE GUIDANCE HAS TRENDED TO THE MORE AMPLIFIED SOLUTION THAT THE ECMWF WAS ADVERTISING YESTERDAY, WITH GOOD OVERALL MODEL AGREEMENT THROUGH THE MAJORITY OF THE FORECAST PERIOD. THE EXCEPTIONS ARE THE UKMET, WHICH BECOMES MORE AMPLIFIED WITH THE TROUGH AFTER 36 HOURS, AND THE NAM IS CLOSEST TO THE NORTHEAST COAST WITH THE TRACK OF THE SURFACE LOW. SECOND SHORTWAVE CROSSING GREAT LAKES AND NORTHEAST THIS WEEKEND ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z GFS/12Z EC MEAN/00Z ECMWF BLEND CONFIDENCE: NEAR AVERAGE SOME VORTICITY SINKING SOUTH ACROSS CENTRAL CANADA FROM THE POLAR VORTEX ON THE BACK SIDE OF THE LARGE SCALE TROUGH WILL SUPPORT A CLOSED LOW NEAR THE NORTHERN GREAT LAKES BY THE END OF THE FORECAST PERIOD SATURDAY MORNING. AN ARCTIC FRONT CAN BE EXPECTED ACROSS THE UPPER MIDWEST AND GREAT LAKES ALONG WITH THE HEIGHT FALLS FROM THIS LOW. THERE IS DECENT TIMING AGREEMENT AMONGST THE MODEL SOLUTIONS, WITH THE UKMET TRENDING SHARPER WITH THE TROUGH AXIS COMPARED TO THE MODEL CONSENSUS. OTHERWISE, THE RELATIVELY SMALL SPREAD REMAINS IN REGARD TO SHAPE AND ORIENTATION OF THE TROUGH AXIS, WITH GOOD AGREEMENT AMONG THE EC MEAN, ECMWF, AND GFS. LARGE CLOSED LOW OFF PACIFIC NORTHWEST COAST BY THIS WEEKEND ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: 00Z GFS/00Z ECMWF BLEND CONFIDENCE: AVERAGE AMPLIFICATION OF STRONG NORTHERLY FLOW ALONG THE ALASKA/YUKON BORDER BEGINS TO CONSOLIDATE INTO A LARGE UPPER LEVEL VORTEX ACROSS THE SOUTHEAST GULF OF ALASKA BY FRIDAY NIGHT, WHERE AN UPPER LEVEL RIDGE PREVIOUSLY RESIDED. THIS WILL SUPPORT INCREASED SOUTHWESTERLY FLOW OUT OF THE SUBTROPICS WITH AN ATMOSPHERIC RIVER AIMED AT THE CALIFORNIA COAST THIS WEEKEND. THERE IS GOOD OVERALL MODEL AGREEMENT WITH RESPECT TO THE SYNOPTIC SCALE NATURE OF THIS VORTEX THROUGH THE FORECAST PERIOD. DIFFERENCES ARE NOTED WITH RESPECT TO THE SHORTWAVE ENERGY ROTATING AROUND IT AND THE ORIENTATION OF THE VORTEX. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... Hamrick